1. Field of the Invention
The present invention relates to a memory effect electroluminescent polychromatic display of the flat screen type usable in the optoelectronics field for the color display of complex images or pictures or for the color display of alphanumeric characters.
2. Brief Description of the Prior Art
It is said that a display has a memory effect if its electrooptical characteristic (luminance - voltage curve) has a hysteresis. For the same voltage within the hysteresis loop, the display can consequently have two stable states, i.e. extinguished/off or illuminated/on.
A memory effect display has significant advantages. In order to display a fixed image, it is sufficient to simultaneously and continuously apply a so-called maintenance voltage to the complete screen. The latter can be a sinusoidal signal or can be in square wave form and in particular the shape and frequency of the maintenance signal can be chosen independently of the complexity of the screen and in particular the number of display points. Thus, in principle, there is no limit to the complexity of a memory effect display screen. Thus, bistable plasma screens with alternative excitation are commercially available having 1200.times.1200 image points or pixels.
In addition, the technology of displays using thin film electroluminescence and capacitive coupling has now reached the final development stage. These displays can be given a so-called inherent memory effect, but this leads to a significant deterioration of the electrooptical performance characteristics. A more attractive method consists of connecting a photoconductive structure (PC) in series with an electroluminescent structure (EL), wherein the structures are optically coupled to one another.
This makes it possible to produce an extrinsic memory effect, which is called the PC-EL memory effect and is based on the following principle. When the display is in the off state, the photoconductive material is not very conductive and retains a significant part of the voltage applied. By increasing V to a value Von, a voltage applied at the terminals of the electroluminescent structure which exceeds the electroluminescence threshold, the PC-EL means switches into its on state. The photoconductive material is then illuminated by and electroluminescent structure and passes into the conductive state. The voltage at its terminals drops and this leads to an increase in the voltage available for the electroluminescent structure. In order to extinguish a PC-EL means, it is merely necessary to reduce the total voltage V to a value Voff below Von, so that a luminance - voltage characteristic having a hysteresis is obtained.
A monochromatic PC-EL structure was recently described in FR-A-2 574 972 and in the Article by the present inventor entitled "Monolithic Thin-Film Photoconductor-ACEL Structure with Extrinsic Memory by Optical Coupling" and published in IEEE Transactions on Electron Devices, vol. ED-33, no. 8, August 1986, pp. 1149-1153.
This structure is diagrammatically shown in section in FIG. 1. It comprises a glass substrate 10 on which are deposited an electrode 12, a first dielectric layer 14, an electroluminescent layer 16, a second dielectric layer 18, a photoconductive layer 20, a third dielectric layer 21 and finally an electrode 22. Electrodes 12 and 22 are connected to an a.c. voltage source 24. In this case the PC and El layers are thin films with a thickness of approximately 1 micrometer.
Such a structure can be simply produced, because it does not require supplementary etching stages. Moreover, the current - voltage behavior of the thin film photoconductor in the dark is highly non-linear and reproducible. The beneficial consequences are that the electrical illumination of the means is always easy, the hysteresis is only slightly dependent on the exciting frequency and the reproducibility of the hysteresis margin between the individual production runs is ensured.
Unfortunately this electroluminescent structure only permits a monochromatic display and at present there are no polychromatic displays using the PC-EL effect.
Thus, the known polychromatic display electroluminescent devices are of two types. The first solution which has been intensively researched in order to obtain polychromatic screens consists of developing an electroluminescent phosphor with an emission spectrum covering at least the red, green and blue and called a "white." phosphor, which is combined with an array of colored filters in order to produce red, green or blue emission pixels in an identical manner to liquid crystal polychromatic screens. This solution is more particularly described in the Article by C. Brunel and N. Duruy, Opto, no. 43, March/April 1988, pp. 30-35, "Colour in Flat Electroluminescent Screens". However, the luminance obtained with such polychromatic screens is well below the levels required for applications, due to the inadequate performance characteristics of the white phosphors.
The second solution is described in the aforementioned Article by Brunel and Duruy and in the Article by Christopher N. King et al, "Full-color 320.times.240 TFEL display panel", pp. 14-17, Eurodisplay, London, Sept. 15-17, 1987. It is diagrammatically shown in section in FIG. 2.
This solution consists of using a first structure incorporating a transparent substrate 30 equipped with an electroluminescent layer 32, which is rendered transparent or semitransparent by an appropriate choice of rear electrodes 34, the front electrodes 36 being transparent. With this first structure is inserted a second, so-called "inverted" structure having a transparent substrate 38 equipped with an electroluminescent layer 40 and transparent electrodes 42 and 44. The first structure has a monochromatic or dichromatic emission spectrum and the second structure a monochromatic emission spectrum complementary of the spectrum of the first structure. This gives a dichromatic or trichromatic display.
The dichromatic structure is obtained by the juxtapositioning of two monochromatic electroluminescent materials emitting different, etched colors (e.g. red and green).
The two structures are controlled separately, but simultaneously in the manner described in SID 86 Digest, pp. 25-28, Article entitled "Multicolor TFEL Display and Exerciser", by W. A. Barrow et al. In this display, the luminance is much too weak for the envisaged application and the voltages and currents used are relatively high.
Moreover, the use of a PC-EL monochromatic display under intense ambient illumination can lead to a significant deterioration of the PC-EL hysteresis. Thus, illumination by an intense external source of the photoconductive layer can lead to reduction in the voltage at the terminals of the latter and consequently to a reduction of the starting voltage. In practice, this leads to the accidental lighting up of certain normally extinguished pixels.
Thus, the invention relates to a memory effect electroluminescent polychromatic display making it possible to obviate these disadvantages.